In many types of microsurgery, particularly ophthalmic retinal surgery, a source of light is required. The light source is used to illuminate the surgical field. In many surgical procedural instances, the surgical field is relatively open and hence focused lamps above the surgical area will suffice to illuminate the surgical field adequately. However, in retinal surgery, the surgical field is inside the globe of the eye. External light sources tend to cause reflections from the cornea of the eye thus distorting the surgical field. Furthermore, when an external light source is used, the aperture available for penetration of the light into the surgical field is limited by the pupil of the eye. For these reasons, it is common practice to use a fiber optic instrument to deliver light to the inside of the eye. By use of such fiber optic instruments, corneal reflections and/or burns are substantially eliminated and the light may be pointed in any desirable direction.
The amount of light required to allow adequate viewing of the surgical field within the eye is considerable. There are several factors which contribute to this. For instance, the inside of the eye does not readily reflect light to surrounding areas. Even though a healthy cornea may be present, there will be some reflection from its inside surface. The same situation applies to the lens. The available light for viewing is reduced. With respect to a cataractous eye or when an unhealthy cornea is present, some of the light is absorbed and/or scattered. In instances where the eye has experienced a hemorrhage, the fluid of the eye, known as the vitreous humor, will tend to be cloudy. This also will reduce the usable light for visualization.
In many instances, a surgeon operates using a binocular microscope whereby the magnification of the surgical sight can be as high as twenty-five times (25.times.). Generally in hospitals, the microscope will have other observation ports for an assistant to see and/or for a video camera. As a result, the apparent brightness to the surgeon is reduced considerably.
To overcome the above difficulties, fiber optic instruments have been developed such that the instrument producing the light casts the light directly on the surgical site. Sometimes it has been found desirable to combine the fiber optic instrument with another instrument. Such combinations can allow the surgeon to use both hands to manipulate the tissues instead of one hand to manipulate tissues and the other to direct the light. These multi-function instruments generally use a smaller optical fiber.
The incision made for insertion of the instrument is as small as possible. Most light sources available for illumination of the optical fibers have a focal point that is much larger than even the largest optical fiber in use. Consequently, the output of the fibers is directly proportional to the cross sectional area of the fiber. In other words, in a fiber having a diameter of 0.03", the light output will be 2.25 times the output of a fiber having a diameter of 0.02". The just mentioned fiber sizes are exemplary of typical fiber sizes used for illuminated optical fibers.
During surgery, the surgeon relies upon the color of the different parts of the eye to diagnose either the condition of the tissue or perhaps the effectiveness of a prior treatment. For this reason, it is important that the color of the light is white. In fact, if the light is slightly yellow or slightly blue, the variance from the white light will tend to color the tissues making it difficult to provide an accurate diagnosis.
Developments have enhanced the intensity of the white light used to illuminate optical fibers. Too high an intensity of light delivered to the eye can result in phototoxicity to the eye. Although studies have addressed the problem of phototoxicity to the eye, specific acceptable levels of intensity have not been identified. Consequently, it becomes more important to have the ability to adjust the intensity of the light by dimming the light without distortion of the light. In order for a method of dimming the light to be satisfactory, it would be necessary for a method to allow a change in the intensity of at least fifty percent (50%) as well as retaining the white light color of the light as if no dimming had been made. The angle of the light impinging on the optical fiber directly affects the output angle of the light from the illuminated fiber. Thus, the aperture of the source of light for illumination of the fiber should remain the same even if the intensity of the light produced by the optical fiber is dimmed.
Several attempts have been made to dim the light intensity available to the optical fiber. One method used by manufacturers is to limit the electrical current available to the source lamp filament. This results in a temperature change of the filament and as a result, the color of the light is changed to a light which is more red. In another attempt, a circular disk is used that is placed perpendicular to the light axis and between the lamp and the optical fiber, the disk has a decreasing width slot in order to decrease the aperture of the source light gradually. Using such a disk causes the light gradually to get blocked by the edges of the slot in the disk. The result is not an actual dimming of the light intensity, but a change in the aperture. The result causes the outer edges of the output circle of the optical fiber to become smeared and then become dark. The center of the output pattern from the fiber is not reduced in intensity.
In still another attempt at dimming the light, a disk is placed in the light path which disk has a series of holes etched through it, as the disk is turned, the density of the holes decreases. The result of using such a method is creation of a series of dark concentric rings in the output of light by the optical fiber. As the disk is turned, the location of the rings changes but the rings never disappear. Here again, the intensity of the light outside of the these rings does not change as the "dimming" mechanism is used. This is caused by a disproportionate blocking of light rays at various angles to the fiber axis. The disk will block more rays at a given angle to the fiber as evidenced by the dark rings in the output. Since the actual hole spacing changes circumferentially, the angular relationship changes as the disk is turned causing the apparent motion of the rings as the source of light is dimmed. Another method of attempting to dim the light source is by moving a light source lamp away from the fiber end. This results in an aperture change because as the lamp is moved away, the output angle narrows.
The present invention provides a dimmer which has no effect on the aperture of the light source for illumination of the optical fiber. Hence, the dimmer of the present invention truly dims the light intensity provided by the illuminated optical fiber.